Method of preparing phosphine



Nov. 5, 1963 I. GORDON 3,109,791

METHOD OF PREPARING PHOSPHINE FiledJuly 2'7, 1960 Zl-CATHODE United States Patent 3,109,791 METHGD 0F PREPARING PHOSPHIINE Irving Gordon, Niagara Falls, N.Y., assignor to Hooker Chemical Corporation, Niagara Falls, N.Y., a corporation of New York Filed July 27, 1960, Ser. No. 45,664 13 Claims. (Cl. 204-101) This invention relates to the preparation of phosphine by the electrolysis of phosphorus.

Heretofore, phosphine has been prepared by the reaction of metallic phosphides or phosphonium halides with Water, and by the hydrolysis of elemental phosphorus. These methods have been unsatisfactory because of the high production costs and/or because the phosphine product is in an impure form.

United States Patent No. 1,375,819, issued April 26, 1921, to Henry Blumenberg, Jr., discloses a method for preparing arsine by the electrolysis of a salt or oxide of arsenic in the presence of sulfuric acid and potassium sulfate or other compounds capable of liberating nascent hydrogen upon electrolysis. iowever, phosphine is not produced under the conditions set forth by Blumenberg when an oxide or salt of phosphorus is employed.

W. R. Grove, in the Journal of the Chemical Society, volume 16 (1863), pp. 263-272, discloses the use of an electric current to boil moist molten phosphorus and produces phosphine thereby. Such a technique requires a high voltage, and converts only a small amount of phosphorus to phosphine.

It is an object of this invention to provide a method of producing phosphine by electrolytic means.

Another object of the invention is to provide a more economical method of producing phosphine.

Still another object of the invention is to provide a method of producing phosphine in a form substantially free from phosphorus hydrides and other phosphorus impurities.

These and other objects of the invention will be apparent from the following detailed description of the invention.

It has now been discovered that phosphine can be prepared by passing an electric current between an anode and a cathode in contact with an electrolyte containing molten phosphorus, while maintaining the molten phosphorus in agitated contact with the cathode.

The accompanying drawing is a schematic illustration of a suitable electrolytic cell for carrying out the novel process.

Referring to the drawing, there is shown a cell vessel containing molten phosphorus 11 in the bottom portion thereof, the phosphorus level being indicated by the interface 12, and an electrolyte 13 contained above the molten phosphorus, the upper level of the electrolyte being indicated by the interface '14. Gas tight cover 15, having ports 16, 17, 18, 19 and 29, is secured to the top of cell vessel 10.

Extending through port '16 is cathode 21, shown in the drawing as a helical coil positioned with its central axis vertically disposed, the coil extending from the bottom of cell vessel 10 up to a level just below interface 14. As will be discussed more fully hereinafter, cathode 21 can be of any shape having a high surface area per unit of weight.

Extending through port 17 is a porous anode chamber 22, having pores 23 below the interface 14, a gas discharge port 24 'above cover 15, and an anode port 25 above cover 15. The bottom portion of anode chamber 22 is positioned above interface 12. Anode 26 extends through anode port 25 into electrolyte 13, below interface 14, but above interface 12. Electric conductors 27 "ice and 28 connect the anode and cathode to the positive and negative poles respectively, of a source: of electrical energy 29.

Gas generated in the anode chamber 22 is discharged through gas discharge port 24. Gas generated in the "cathode chamber (the portion of the cell vessel 10 outside of the anode chamber) is discharged through gas discharge port 30' which extends through port 18.

A fresh supply of molten phosphorus and/or electrolyte may be introduced into cell 10 by means of tunnel 31, which extends through port 19.

A thermometer 41 or other suitable temperature measuring means extends through port 20' into the electrolyte.

Cell vessel 10 is placed in a constant temperature bath 32 having a fluid inlet means 33 and fluid outlet means 34. The constant temperature bath 32 is filled to level 35 with a suitable fluid such as water.

Cell vessel 10 is provided with a suitable agitation means 36, positioned in the molten phosphorus 11. The agitation means 36 may be a magnetic stirrer as shown, but any suitable agitation means may be employed. Magnetic core 37 is preferably covered with a suitable corrosion resisting layer 38. Magnetic core 3 7 is caused to notate by impressing an electric current on a rotatable magnetic core 39, positioned in the magnetic energy source 40' of the agitating means 36. Agitation is suflicient to maintain the molten phosphorus in agitated contact with the cathode and to disperse a portion of the molten phosphorus in a portion of the electrolyte, a portion of the resulting dispersion being in agitated contact with the cathode.

It will be recognized by those skilled in the art that the design of the electrolytic cell shown in the drawing can be modified without departing from the spirit of the invention. For example, the position of the anode and the cathode can be reversed, lead coil cathode 21 being positioned in a unit similar to porous anode chamber 22, and anode 26 being positioned in the electrolyte on the opposite side of the diaphragm. In such a cell the molten phosphorus is placed inside of the porous diaphragm, rather than at the bottom of the cell vessel. In such a case, stirring of the molten phosphorus may be effected by an overhead motor driven stirrer rather than the magnetic stirrer shown in the drawing.

Regardless of the type of cell employed, it is important that sufficient agitation be imparted to the molten phosphorus so that a relatively thin layer of molten phosphorus is applied to the largest possible area of the cathode.

The cell vessel may be constructed of glass, ceramics, rubber-lined steel, or other impervious materials.

Porous anode chamber 22 may be constructed of any suitable porous material, such as sintered glass, plastic cloth constructed of fibers of tetrafiuoroethylene polymer, glass cloth, porous alundum, ion-exchange membranes and the like.

Any solid material having a hydrogen over-voltage as normally measured in the absence of phosphorus exceed ing the hydrogen overvoltage of smooth platinum may be employed as the cathode. Typical cathodic materials include lead, lead-mercury amalgam, tin, cadmium, copper, bismuth, aluminum, zinc, brass, silver, nickel, tellu-rium, Monel, gold and alloys thereof. For example, the alloy known as Woods metal which is an alloy containing fifty percent bismuth, twenty-five percent lead, twelve and one-half percent tin, and twelve and one-half percent cadmium may be employed. Black phosphorus may also be employed as a cathode material. It is desirable to employ 'a cathode in a form having a high unit of area per unit of weight. For example, as shown in the drawing, the cathode has the form of a helical core, but wire l 3 mesh screen, perforated sheets or cylinders, and other suitable forms may be employed.

Suitable anode materials include lead, lead-antimony,

lead dioxide, platinum, graphite and stainless steel, 7 Any electrolyte which is non-reactive with molten phosphorous, and which is capable of forming hydrogen ions under the electrolysis conditions employed, may be employed as the electrolyte. Suitable electrolytes include aqueous solutions of phosphoric acid, sulfuric acid, hydrochloric acid, sodium chloride, lithium chloride, sodium sulfate, potassium chloride, potassium sulfate, monosodium phosphate, disodium phosphate, monopotassium phosphate, dipotasium phosphate, and mixtures thereof. An aqueous phosphoric acid solution containing between about ten and about eighty, and preferably between about fifteen and about fifty percent phosphoric acid by weight, is preferably employed as the electrolyte, but other concentrations may be employed if desired. Concentration of the aqueous solutions of the aforesaid acids, when employed an an electrolyte, should be equivalent to the aforesaid phosphoric acid concentration. Aqueous solutions of the aforesaid salts having a concentration between about ten percent by weight and the concentration suffi cient to produce a saturated solution under the temperature conditions obtained, may be used.

If phosphoric acid, sulfuric acid, or salts of these acids are employed as the electrolyte, the anolyte gas predominates in oxygen, when other than graphite is employed as the anode. If these acids and/ or salts are employed as the electrolyte,'and the anode is constructed of graphite, the anolyte gas will predominate in carbon dioxide.

Molten white phosphorus, sometimes referred to as yellow phosphorus, is preferably employed as the source of phosphorus for the production of phosphine, but other allotropic forms of phosphorus may be employed if desired. The temperature of the phosphorus should be sufficient to maintain it in a molten state, without effecting boiling thereof. For this reason, the temperature of the molten phosphorus and electrolyte is maintained within the range between forty-four degrees and about two hundred and eighty degrees centigrade, and preferably between about iifty and about one hundred and twenty degrees centigrade. Temperature control of the phosphosphorus and'the electrolyte is readily obtained by means of constant temperature bath 32, but any suitable temperature control means may be employed. For example, on start-up of the electrolytic process, the molten phosphorus and electrolyte may be heated to a temperature within the aforesaid temperature range by means of an external source of heat, and maintained at this temperature by means of a constant temperature bath. 7

When an electriccurrent is impressed on the system,

and the molten phosphorus is maintained in agitated contact with the cathode, gaseous phosphine and hydrogen are formed at the cathode and discharged through gas discharge port 30. The anolyte gas will vary, depending upon the composition of the electrolyte and the anode, as discussed above.

Control of the current and current density during electrolysis is important. The current should be maintained at at least one ampere or above, preferably as high as possible without increasing the voltage above about twentyfive volts. Current density is preferably maintained as high as possible, consistent with a reasonable level of phosphine production. Commercially acceptable phosphine production rates can be attained when the cathodic current density is maintained at least about five amperes per square foot and preferably between about ten and or higher. When an acid is used as the electrolyte, the resulting catholyte gas is substantially free from phosphor'us hydride impurities. As a result when the catholyte gas is employed as a chemical intermediate, the product of the reacton is in a highly pure form. For example, tetrakis (hydroxymethyl) phosphonium chloride is prepared by the reaction of phosphine, formaldehyde, and concentrated hydrochloric acid. When phosphine prepared by conventional processes is employed to produce tetrakis (hydroxymethyl) phosphonium chloride, the resulting product has a purity of about ninety-six point five percent. In contrast, when phosphine prepared in accordance with the instant novel process is used to prepare tetrakis (hydroxymethyl) phosphonium chloride, the pu-' rity of the product is as high as'ninety-nine point nine percent.

The following examples are presented to define the invention more clearly without any intention of being lim ited thereby. All parts and percentages are by weight unless otherwise specified.

Example 1 A cell of the design shown in the drawing with the exceptions noted hereinafter, was used to produce phosphine in this and subsequent examples. The volume of the cell vessel was about five hundred milliliters. A lead disk about one-quarter inch thick and about three inches in diameter was employed as the cathode. A graphite rod, three-eighths of an inch in diameter and approximately six inches long, was employed as the anode. A selenium-type rectifier was employed to convert sixty cycle one hundred and ten volt alternating current to direct current. A voltage of thirteen volts and a current of one ampere was employed during the electrolysis.

Fifty-three grams of molten phosphorus was placed in the cell vessel, and about four hundred milliliters of an aqueous phosphoric acid solution-containing eighteen percent phosphoric acid was added to the cell vessel. The temperature of the electrolyte and molten phosphorus was maintained at about forty-six degrees centigrade.

Agitation was effected by means of a Teflon coated magnetic stirrer, which was rotated at the rate of approximately two hundred revolutions per minute. When the cell was energized, carbon dioxide was produced at the anode and a phosphine-containing gas was produced at the cathode. Analysis of the cathode gas showed that it contained fifteen percent phosphine by volume, and

that this gas was produced at the rate of five milliliters per minute.

Example 2 The procedure of Example 1 was repeated, employing a Woods metal disk as the cathode, instead of the lead disk of Example 1. Thirty grams of molten phosphorus about seven hundred amperes per square foot. However,

was placed in the cell vessel, and about four hundred milliliters of the aqueous phosphoric acid solution of Evample l were added thereto. A voltage of 17.5 volts, and a current of two amperes were impressed upon the system. Electrolysis was carried out at a temperature of forty-eight degrees centigrade. The gas produced at the cathode contained 4.5 percent phosphine by volume, and this gas was produced at a rate of 3.9 milliliters per minute.

Example 3 The procedure of Example 1 was repeated employing a cadmium disk as the cathode instead of the lead disk of Example 1. A tungsten tube type rectifier'was employed as the source of electrical energy. Thirty grams of m0lten phosphorus were added to the cell vessel, and four hundred and fifity milliliters of the aqueous phosphoric acid solution of Example 1 were also added thereto. A voltage or" twenty-three volts and a current of two amperes were impressed upon the system. The molten phosphorus and electrolyte were maintained at a temperature of eighty degrees centigrade during electrolysis. The cathode gas was produced at the rate of 12.2 milliliters per minute and contained 39.5 percent phosphine by volume.

Example 4 The procedure of Example 1 was repeated employing a helical coil of one-eighth of an inch in diameter lead wire as the cathode, and a tungsten tube type rectifier as the source of electrical energy. The coil was six inches high, two and one-half inches in diameter and had an effective cathodic area of 43.2 square inches. One hundred grams of phosphorus and three hundred and fifty milliliters of the aqueous phosphoric acid solution of Example 1 were placed in the cell vessel and agitated. The temperature of the phosphorus and electrolyte was maintained at eighty-five degrees centigrade. A voltage of 17.2 volts and a current of three amperes were impressed upon the system. G-as was produced at the cathode at the rate of 17.1 milliliters per minute. This gas contained 57.9 percent phosphine by volume.

Example 5 The procedure of Example 4 was repeated employing a platinum gauze anode, which was two inches long and one-half inch outside diameter, and had an effective ano-dic area of about 3.14 square inches, to replace the graphite "anode. A current of three amperes and a voltage of 18.7 volts were impressed upon the system. A gas containing sixty percent phosphine by volume was produced at the cathode at the rate of 17.9 milliliters per minute.

An important advantage in preparing phosphine in accordance with the instant novel process, is that the gas is free of other phosphorus hydrides and therefore is not spontaneously flammable when in contact with air.

It will be recognized by those skilled in the art that various modifications within the invention are possible, some of which have been referred to above. Therefore, I do not wish to be limited except as defined by the appended claims.

I claim:

1. The process for the production of phosphine which comprises passing an electric current between an anode and a solid cathode in contact with an aqueous electrolyte containing molten phosphorus, said molten phosphorus being in agitated contact with said cathode, whereby phosphine is produced at the cathode.

2. The method of claim 1 wherein the cathode is a solid metal having a hydrogen .ovenvoltage as normally measured in the absence of phosphorus exceeding the hydrogen overvoltage of smooth platinum.

3. The process of claim 1 wherein said cathode is lead.

4. The process of claim 1 wherein said cathode is cadmium.

5. The process of claim 1 wherein said cathode is an alloy of bismuth, lead, tin and cadmium.

6. The process of claim 1 wherein said electrolyte is capable of forming hydrogen ions under the electrolysis conditions employed, and is non-reactive with molten phosphorus.

7. The process of claim 1 wherein said electrolyte is phosphoric acid.

8. The process of claim 1 wherein said electrolyte is an aqueous phosphoric acid solution containing between about ten and about eighty percent phosphoric acid by weight.

9. The process of claim 1 wherein the temperature of the electrolysis and molten phosphorus is maintained during electrolysis within the range between about fortyfour and about two hundred and eighty degrees centigrade.

10. The process of claim 1 wherein the temperature of the electrolyte and molten phosphorus is maintained during electrolysis within the range between about fifty and about one hundred and twenty degrees centigrade.

11. The process of claim 1 wherein the current density of said cathode during electrolysis is maintained in the range between about ten and about seven hundred amperes per square foot.

12. The process of claim 11 wherein the voltage during electrolysis is maintained below about twenty-five volts.

13. The process for preparing phosphine which comprises passing an electric current between an anode and a cathode in contact with an aqueous phosphoric acid solution electrolyte, a portion of said electrolyte in contact with said cathode being admixed with molten phosphorus, said molten phosphorus being in agitated contact with said cathode, and maintaining a current density on said cathode of at least about five amperes per square foot, whereby a phosphine-containing gas is produced at the cathode.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Ephrain: Inorganic Chemistry, 5th ed. (1948), pages 617-622.

Pauling: College Chemistry, 1955, pages 330335.

Creighton et al.: Electro-Chemistry, 2nd ed., 1944, page 47.

Journal of the Chemical Society, volume 16 (1863), pages 263-272. 

1. THE PROCESS FOR THE PRODUCTION OF PHOSPHINE WHICH COMPRISES PASSING AN ELECTRIC CURRENT BETWEEN AN ANODE AND A SOLID CATHODE IN CONTACT WITH AN AQUEOUS ELECTROLYTE CONTAINING MOLTEN PHOSPHORUS, SAID MOLTEN PHOSPHORUS BEING IN AGITATED CONTACT WITH SAID CATHODE, WHEREBY PHOSPHINE IS PRODUCED AT THE CATHODE. 